The way optical networks are used is undergoing significant change, driven in part by the huge growth of traffic such as multimedia services and by the increased uncertainty in predicting the sources of this traffic due to the ever changing models of content providers over the Internet. Sophisticated modulation schemes for higher bandwidth 100 Gb/s services and beyond are known and come into commercial use in optical networks of large and increasing link and node numbers. A bottleneck to widespread deployment of such schemes is the “fixed” wavelength grid approach specified by the International Telecommunication Union (ITU), in which the relevant optical spectrum range in the C-band is divided into fixed-sized spectrum slots. Such conventional “fixed grid” WDM (wavelength divisional multiplexed) networks work on the concept of a fixed spectrum grid typically with a spacing of typically 50 GHz between channels with 80 to 100 of these channels per fiber. In these networks, an individual signal serving a demand between two nodes in the network has to keep within one of these channels or slots defined by guard bands, as otherwise the signal becomes notched and degraded by the wavelength filters. As a result of this restriction, advanced modulation formats allowing up to 100 Gbit/s per 50 GHz channel commercially and up to 200 Gbit/s experimentally, have not to now been usefully deployed in a widespread manner. This is because the spectral widths of such signals are wider than can be accommodated within the 50 GHz fixed grid spacing, so the potential of additional increases in transmission speed cannot be realized.
As used herein, a “slot”, “wavelength” or “channel” is defined as a wavelength or a spectrum of wavelengths associated with a certain signal size. A “carrier” carries a “signal” or “demand” in the known fashion. As is also known, a connection between nodes is made by assigning spectral (i.e. wavelength) slots on the optical links comprising the path between source and destination.
A response to the problems posed by the decade-old ITU fixed grid approach is the flexible grid or “flexgrid”, which facilitates a new optical networking paradigm known as EON (elastic optical networking). The EON technologies allow for radically different network design and operation methodologies that can increase the amount of traffic the network can carry compared to conventional WDM networks, but need different processes to make them operate effectively to get the most out of such networks. In the flexgrid approach, the optical spectrum can be divided up flexibly in dependence on requirements, and elastic optical paths (i.e. paths with variable bit rates) can be generated. This allows for operational and functional flexibility in use of both the optical spectrum and transceivers, previously unavailable in fixed grid implementations. In a flexgrid, the spectrum grid is divided into much finer slot widths, e.g. 12.5 GHz or less, compared to the 50 GHz in the fixed grid approach. Significantly, adjacent channels can be joined together to form arbitrary sized slots to carry signals of a variety of widths, allowing for signals ranging in size from an individual channel to that occupying the entire optical spectrum to be carried. Representations of the fixed and flexible grid approaches are depicted in the example graphs shown in FIG. 1, in which graph (a) is a depiction of the fixed grid approach, in which guard bands (2) partition adjoining optical channels (4) occupied by demands or wavelengths at a particular bit rate. Graph (b) illustrates the flexgrid approach used in an EON network, in which the demands (here shown to be of various spectral widths) are not constrained within a slot of pre-defined spectral size. As illustrated by demand (4b) in graph (b) of FIG. 1, a high bitrate demand with a spectral width exceeding fixed grid slot sizes can be accommodated.
A “superchannel” (depicted in graph (c) of FIG. 1) for carrying demands which are too large to be handled by a single optical channel, can similarly be accommodated in an EON network. A superchannel comprises a grouping of multiple channels and is handled as a single entity, traversing the network for demultiplexing at the receiver end. Specifically, they can be produced by a bandwidth variable transponder (BVT) (a new, known, technology) which can increase its bandwidth as and when required by increasing its spectrum usage. Specifically, the BVT generates carriers which can be aggregated at the transceiver to produce an optical signal of a size which depends on the level of traffic carried by the signal. So if more traffic needs to be carried by the BVT, additional carriers can be added, and conversely if traffic levels decrease, carriers can be disabled. A general description of the use of BVTs in an EON network can be found in “Elastic Optical Networking: A New Dawn for the Optical Layer?” by O. Gerstel, M. Jinno, A. Lord, S J B Yoo (IEEE Communications Magazine, February 2012). The operational flexibility of superchannels can usefully cope with growth in traffic levels in a network over time, and significant spectral savings can be gained over the fixed grid approach. BVTs can be used in both fixed grid and flexgrid systems, although they are deployed to greater effect in flexgrid networks owing to the capacity of the latter to accommodate the greater spectral widths of superchannels.
In a conventional WDM network, an optical transceiver or transponder serves to allocate optical spectrum in the form of a channel of the size needed for transmission of the particular signal of a particular bit rate. In an EON network, a slot of the required spectral width would be established to enable the signal to be added. Where signals are already being carried on the relevant link(s), the transponder would find a section of free spectrum of the required size for the purpose. The addition of new signals in this way onto a flexgrid spectrum could result in the formation of a spectrum interstice between the occupied sections, because new carriers/demands are added in a piecemeal fashion as long as its spectral width can be accommodated. Over time, a number of isolated slivers of free spectrum between occupied sections are formed, which are too small unable to accommodate a new signal which typically requires a contiguous block of free spectrum. This fragmentation of the spectrum is graphically depicted in graph (b) of FIG. 1, in which two demands (4a and 4b) have been added in a way so that the section of free spectrum between them is unusably narrow. FIG. 2 depicts the operation of nodes within a conventional WDM network, node A (10) is connected to node B (12) by an optical link (18) comprising spectral resource. In the known manner, the setup in both nodes are functionally similar to each other and each includes a multiplexer (20) and transponders (14) allowing signals or demands (4, 8) to be sent by, and received from, each other. During operation, some of the demands (8) originate from elsewhere and are transiting through the link, so they already pre-occupy a part of the link. A new demand (4) which originates from the sending node itself is served by individual transponders (14) within the sending node, and can be added to the link only if there is a gap of sufficient size, i.e. a block of sufficient spectral resource, for it to be added onto the link by multiplexing with the transiting demands (8).
As traffic levels within the network increase over time, more transponders are added as each transponder can serve only one demand at a time, using different parts of the spectrum and possibly different routes across the network. New demands (whether carried on a single channel of a particular width, or on a superchannel) are added in a piecemeal fashion without any overall planning or strategy. This potentially gives rise to ever-greater levels of optical spectrum fragmentation over time, which results in a situation where routes through the network for new demands (4) cannot be established due by blocking from existing demands (8), even if the cumulative sum of the free spectrum slivers is sufficient for the purpose. A solution at the optical layer to reduce the level of spectrum “entropy” comprising such fragmentation or disorder in an optical resource during operation, is set out in the applicants' co-pending application EP13250053.9. Here, a routing method is proposed with the aim of reducing waste of optical bandwidth, which uses an entropy measure of the link(s), route or network carrying the demand. In this approach, referring to graph (b) of FIG. 1 for example, demands (4b and 4c) are packed in closely to minimize entropy levels so that a contiguous section of free spectrum is left available for addition of new future demands.
Methods of routing traffic based on dynamic demands are known from e.g. US 2007195700, which describes an approach from the perspective of higher networking layers (such as MPLS) and not at the optical layer. US 2008056717 relates to a routing and wavelength assignment in fixed grid network using fixed bandwidth transceivers but taking account of physical layer impairments (e.g. attenuation and polarization mode dispersion) in determining an end-to-end route. This approach follows a system of adding new wavelengths to carry additional traffic being put on the network as and when the requests come in and these could take different routes through the network.
In the case of superchannels (which term shall include those signals or demands which can vary in spectral size over the time) however, the problem related to spectrum availability is different. It is more concerned with the uncertainty of there being sufficient available optical resource on the link to cope when the growth in the superchannel size or demand occurs. There is a need to address the above issues in connection with the manner in which spectrum-related issues can be minimized in their impact on the growth of the spectral width of optical channels in flexgrid implementations in a WDM-based network.